1. Field of the Invention
The invention concerns a method for continuous monitoring of dynamic loads, including stresses and strains in large hull structures for vessels, according to the introduction to claim 1. The invention also concerns a strain measurement system for implementing the method, comprising an optical transmitter unit, an optical receiver unit and a fiber optic cable network, wherein the transmitter unit and the receiver unit are provided in a central monitoring unit which is connected via an interface with a computer-implemented control system which in turn is connected to a display and data presentation unit and possibly other measurement systems. Finally, the invention also concerns a temperature-compensated, polarimetric, fiber optic strain sensor for use in a strain measurement system.
2. Description of the Related Art
It is a well known fact that large hull structures, such as both ship and aircraft hulls, are frequently exposed to substantial, dynamic loads. These loads can result in damage to the hull and lead to fatigue and fracture, often with disastrous consequences. Load damage to hull structures normally only becomes apparent in connection with a regular service and inspection carried out by the application of various non-destructive test methods. The problem is that load damage to a hull structure can occur when the vessel is underway and develop with disastrous consequences before it can be discovered in the normal inspection.
From British patent application GP 2238112 (U.S. Pat. No. 5,038,618) a method is known for detection and monitoring of strains in large, technical structures by means of fiber optic cables which are attached to the structure. An optical signal is transferred along the cable to an optical receiver. By analysing the optical signal any changes with regard to length and/or changes in the length of the cable can be shown. The change in the cable length is found by determining phase changes on one or more modulation frequencies, by means of interferometry or pulse time measurement. By means of this method a ship's hull or the like can be monitored in order to indicate strains, but bending or twisting in various beam-like structures can also be revealed.
The method according to this patent publication substantially only provides the opportunity of monitoring global strains, i.e. the relative strains between two points which form the respective end points of the fiber optic detection cable. Even though the awareness of global strains is naturally important in large, dynamically-loaded structures, a more localized strain monitoring is required in order to be able to follow the development of the progress of stress or strain and not least at those individual points in a structure which in all probability will be more vulnerable to strain than other points, and a local strain monitoring will therefore be essential in order to maintain safety. At the same time data from a number of local strain sensors, possibly in connection with one or more global strain sensors and a system which records the forces to which the structure in its entirety is exposed, can be employed in order to provide a complete strain history for the structure, and this can be done with high resolution both in the temporal and spatial domains.
This, however, is based on the assumption that the strain system fulfils specific technical requirements which as yet have not been easy to implement in known, fiber optic strain measurement systems, whether they are based on phase change, interferometry or pulse time measurement. Problems with regard to accuracy and resolution become particularly critical if advantageous data from a large number of local sensors arranged in a very large structure, such as, e.g., a large bulk carrier or a tanker, have to be collected and transferred to a central monitoring unit in real time.